1. Field of the Invention
The present invention relates to a developer composition for electrostatic image development in electrophotography, electrostatic recording, electrostatic printing and other fields, and more particularly to a developer composition suitable for flash fusing.
2. Discussion of the Related Art
As described in U.S. Pat. Nos. 2,297,691 and 2,357,809 and other publications, the prior art electrophotography comprises forming an electric latent image by evenly charging a photoconductive insulating layer and subsequently exposing the layer to eliminate the charge in the exposed portion and visualizing the formed image by adhering colored charged fine powder known as a toner to the latent image (a developing process), transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process), and permanently fixing the transferred image by heating, pressure application or another appropriate means of fixing (a fixing process).
As stated above, a toner must meet the requirements not only in the development process but also in the transfer process and fixing process.
With respect to the fixing process described above, the most common among currently available methods is so-called heat roller fixing, wherein heat and pressure are applied simultaneously, but flash fusing, a fixing method using light, is used for the following reasons:
1) Because it is non-contact fixing, there is no fear of image staining, i.e. offset phenomenon, or deterioration of resolution, such as with image expansion upon passage through the upper and lower rollers, as seen in contact fixing; PA1 2) Because it is unnecessary to make a specific design for preventing image staining as seen in contact fixing, for example, addition of wax as a release agent to toner, coating with silicone oil as a release agent to a fixing roller, etc. the degree of freedom in designing the toner and fusing apparatus is high; PA1 3) Fixing is possible irrespective of the material and thickness of recording paper. PA1 (1) an electrophotographic developer composition for flash fusing containing at least a binder resin based on a polyester resin and a colorant, wherein polyester resin is formed between an acid component, not less than 80 mol % of the component being phthalic acid series dicarboxylic acid and an alcohol component, not less than 80 mol % of the component being bisphenol A alkylene oxide adduct, said polyester resin having an acid value of not more than 10.0 KOH mg/g, a softening point of not less than 95.0.degree. C. and not more than 125.0.degree. C., as determined using a flow tester of the "koka-shiki" type, and a glass transition temperature of not less than 50.0.degree. C. and not more than 80.0.degree. C., as determined by DSC, and substantially no odor is produced upon flash fusing; PA1 (2) the electrophotographic developer composition for flash fusing as described in (1), wherein the polyester resin has a number-average molecular weight of not less than 3,000 and not more than 6,000 and a weight-average molecular weight of not less than 10,000 and not more than 500,000; and PA1 (3) the electrophotographic developer composition for flash fusing as described in (1) or (2), wherein the polyester resin adsorbs water in a ratio of not more than 0.7%, as determined by Karl Fischer's method after being kept standing at a temperature of 35.degree. C. and a humidity of 85% RH for 24 hours.
However, while having such advantages, flash fusing is subject to various limitations as to the developer binder used due to the inherent property thereof. Specifically, when the toner used for heat roller fixing is used as such, for instance, a problem arises that high energy is required to soften and fuse the toner because the molecular weight of the binder is so high that it cannot be easily softened and fused; in flash fusing, the toner image formed on the recording paper must be irradiated with flash light using an electrical discharge tube such as a xenon flash lamp, thereby instantaneously raising temperature to soften and fuse the toner and adhere it to the recording paper.
Also, in flash fusing, because the toner is instantaneously heated to a high temperature, a large amount of decomposed product is formed on the toner surface, which in turn contaminates the working environment. In recent years, with the increasing demand for more rapid printing, the amount of decomposed product formed per unit time has increased, thus posing a problem of influence on the working environment. For this reason, a developer for flash fusing with little decomposition product is now desired.
To meet this requirement, there has been proposed a developer for flash fusing based on a polyester resin containing terephthalic acid and/or isophthalic acid as a main acid component and neopentyl glycol as a main alcohol component (Japanese Patent Laid-Open No. 75755/1988). However, this invention is not satisfactory with respect to prevention of influence on the working environment, since the molecular weight of the resin is in the oligomer range so that residual monomer of neopentyl glycol is present, which is potentially odorous upon fixing, though the thermal decomposition resistance of the toner has improved. Also, in the polyester resins based on neopentyl glycol, since the ester group concentration is high and hence a large amount of water is adsorbed in a high-temperature, high-humidity environment, that evaporation of the water contained therein or hydrolysis is induced by the high temperature in the flash fusing, which results in increased fumes attributable to sublimation of the low molecular components.
On the other hand, another developer for flash fusing containing bisphenol A/epichlorohydrin type epoxy resin as the main resin components has been proposed, wherein the amount of the low molecular substances formed upon flash fusing is very low (Japanese Patent Laid-Open No. 30139/1981). However, such a resin undergoes void formation due to its low fusing viscosity, though it is easily and rapidly fusible and fixable on recording paper, even when the intensity of the flash light used is very weak. Here, the term void means the porosity of the fused image, wherein porous fusing of the fused image occurs due to the low fusing viscosity of the resin.
As a method for improving the porosity of the fused image, the use of a terminal amide-modified epoxy resin, which has resulted from the reaction of a low molecular amide compound with an epoxy compound at the molecular terminal thereof, as a binder resin has been proposed (Japanese Patent Laid-Open No. 270964/1987). However, this method is unsatisfactory in that the higher fatty acid used, which has 10 or more carbon atoms, tends to be present as a residue, which can be odorous upon exposure to high temperature, though void has improved. Also, another problem arises that attention should be given to the use of epoxy compounds, which have a highly reactive epoxy ring at the terminal thereof, because some of them show positive response in mutagenicity test.